The present invention relates to radiation therapy principally for the treatment of cancer and benign diseases, and in particular, radioactive sources used in one radiation therapy technique termed brachytherapy.
Brachytherapy is radiotherapy where small, radioactive sources are placed into diseased tissues, in body cavities near disease, or in contact or close proximity to disease. Brachytherapy takes many forms, the most common of which are permanent implants, where the sources are left in place from the time of implantation for the life of the patient, or temporary implants, where the sources dwell in the treatment location for a specified time, after which they are removed. The temporary implants may be low dose-rate, where the sources stay in place for times on the order of a day to a week, or high dose-rate, where a treatment takes a matter of minutes. There is also a middle dose-rate region where treatments range from a couple of hours to just over a day.
One common example of a permanent implant occurs with treatment of cancer of the prostate. Prostate permanent implant brachytherapy is a radiation treatment technique in which radioactive sources are implanted directly into the prostate and left in place permanently. Typically, 50 to 100 small radioactive sources are implanted near the tumorous tissue.
The sources may be radioactive material absorbed onto small resin spheres contained within a titanium capsule or on the surface of a silver rod also sealed in titanium. The sources often use iodine-125 as the source material for the radiation, which has a half-life of approximately sixty days providing an average energy of emitted photons of approximately 27 keV, with commercial source strengths in the range of 0.2-1.0 mCi.
The sources, of a size 0.8 mm in diameter and 4.5 to 5 mm long, may be implanted using a hollow needle. The needle provides a lumen 1.3 to 1.5 millimeters in diameter and about 20 cm long into which the sources may be inserted along with spacers controlling their separation. The loaded needle is inserted into the patient, and then withdrawn, while a plunger ejects the contained sources.
The location of the radioactive sources is desirably selected to provide a prescribed dose to the diseased tissue of the prostate while sparing surrounding sensitive, critical tissue, for example, the urethra and rectum. Source placement in the region between sensitive tissue and diseased tissue is a compromise between providing sufficient dose to the diseased tissue and minimizing dose to the sensitive tissue.
Prostate cancer can also be treated using high dose-rate brachytherapy. In this case, needles are placed into the prostate and then connected to a treatment unit. The unit moves a very intense radioactive source through the needles, stopping at determined positions for times calculated to deliver the desired dose. The device steps the source through the first needle, retracts it and then moves the source through the next needle. This pattern continues until all needles have been accessed by the radioactive source and the treatment is concluded.
Many cancers are treated in a manner between the two described, where needles are placed into the target, and many sources are placed into the needles, with source strengths and positions calculated to deliver the desired dose in the prescribed time. The dose pattern emitted by these sources is isotropic relative to the lengthwise axis of the encapsulated cylindrically shaped source.